The invention is based on a phosphor composition for low pressure gas discharge lamps for generating light with a color temperature of greater than 5000 K and a very good general color rendering index Ra of greater than 90, containing at least one halophosphate phosphor, a phosphor which emits in the red wavelength region and a phosphor which emits in the blue-green wavelength region.
In addition to the requirement for a very good color rendering, in such cases the light flux must be as high as possible. The difficulty in this context is that high light flux and very good color rendering are contradictory requirements, since a high light flux requires a maximum light intensity in the green, but a good color rendering presupposes a distribution of the light intensity which is similar to the black-body radiation at all wavelengths, and is therefore contradictory to the first requirement.
It is an object of the present invention to provide a phosphor composition containing at least one halophosphate phosphor, a phosphor which emits in the red wavelength region and a phosphor which emits in the blue-green wavelength region, with which, in low-pressure gas discharge lamps, it is possible to achieve maximum light efficiencies combined, at the same time, with a very good general color rendering index Ra of greater than 90 and a color temperature of greater than 5000 K. Moreover, it should be possible for the phosphor composition to be applied to the discharge vessel in one single layer.
This object is achieved by a phosphor composition containing beside the at least one halophosphate phosphor, the phosphor which emits in the red wavelength region and the phosphor which emits in the blue-green wavelength region additionally an Eu-doped barium magnesium aluminate phosphor BaMgAl10O17:Eu and a Tb-doped green-emitting phosphor.
The required features concerning the color temperature of greater than 5000 K and the general color rendering index Ra of greater than 90 are very well fulfilled as a result of the additional use of an Eu-doped barium magnesium aluminate phosphor BaMgAl10O17:Eu and of a Tb-doped green-emitting phosphor in the phosphor composition.
The proportion by weight of the Eu-doped barium magnesium aluminate phosphor BaMgAl10O17:Eu in the overall phosphor composition is advantageously between 1% and 20%, preferably between 3% and 15%.
A particularly suitable Tb-doped green-emitting phosphor is a Tb-doped cerium magnesium aluminate phosphor CeMgAl11O19:Tb. The proportion by weight of the Tb-doped cerium magnesium aluminate phosphor CeMgAl11O19:Tb in the overall phosphor composition should be between 1% and 20%, advantageously between 2% and 15%.
To further improve the color rendering, the phosphor of the phosphor composition which emits in the blue-green radiation region should be an Eu-doped strontium aluminate phosphor Sr4Al14O25:Eu, the proportion by weight of the Eu-doped strontium aluminate phosphor Sr4Al14O25:Eu in the overall phosphor composition advantageously being between 10% and 50%, preferably between 20% and 40%.
Further improvements with regard to the color rendering can be achieved by adding an Mn-doped cerium gadolinium zinc magnesium pentaborate phosphor (Ce,Gd)(Zn,Mg)B5O10:Mn for the phosphor which emits in the red radiation region, in which case this phosphor should form from 5% to 60%, preferably from 10% to 40%, of the overall phosphor composition.
A particularly suitable halophosphate phosphor for the features required here is an Mn-doped calcium halophosphate phosphor Ca5(PO4)3(F,Cl):Sb,Mn.
In addition, the phosphor composition may contain one or more phosphors which emit in the invisible spectral region. In this way, the phosphor composition can be approximate to the spectrum of the sun with regard to its UV component.
The phosphor composition is preferably applied to the inner side of the discharge vessel in the form of a single phosphor mixture and in this case comprises a single layer.
However, it may also be advantageous for the phosphor coating to be applied in the form of two layers, in which case the lower layer, which faces the inner side of the discharge vessel, consists only of the halophosphate phosphor. The covering layer, which faces the discharge, should then likewise contain the halophosphate phosphor, and also the other phosphors of the phosphor mixture.
In addition, a protective layer comprising Al2O3, Y2O3 or a rare-earth oxide should be applied between the inner side of the discharge vessel and the phosphor layer.